I can't think of any fight this would be useful on aside from Gara'Jal if you are progressing on it and your healers are struggling
The bosses just dont do enough sustained DPS for it to be worth it this tier
As from a pure tanking perspective you will get more damage reduction from the increased vengeance dumping it into the occasional barrier anyway:P

Aside from that it could be also useful when it comes to soloing stuff. Other then that: offtank could switch to it on fights that again arent dps heavy (as if a tank without vengeance would do any significant dps anyway) and have phases where u ened 2 tanks but not the whole fight long (bladelord, empress and so on. (not to say that u dont need dps for those fights))
well i see that it probably not worth usuing , but since i just got it i still find it good to know

The attackspeed debuff has no effect on targets above lvl 60. They fixed this during TBC since people started to do these kind of things.

If the debuff would still work on todays bosses one could argue that it would be worth weaponswaping, the sword has a very high chance to proc so just doing a few swings and swaping back would probably be ok.